1. Field of the Invention
The present invention relates generally to the field of Stirling or similar cycle thermal machines and is more particularly directed to a thermal machine sealed within an outer shell enclosure which maintains the engine components in an axially compressed state to thereby simplify construction of the engine while maintaining a sealed atmosphere of working fluid about the engine thereby minimizing leakage seal problems and also providing a containment enclosure about the engine in the event of explosive failure of the same.
2. State of the Prior Art
The present invention is generally directed to thermal machines of the general type wherein a working fluid is subjected to a thermodynamic cycle within a chamber by reciprocating a displacer body within the chamber for displacing the working fluid between a hot and a cold space in the chamber. In the case of an engine a heat input is provided and the resulting cyclic variations in working fluid pressure may drive a work piston so as to derive a work output. Conversely, a work input may reciprocate a compressor piston so as to alternately compress and expand the working fluid. The resulting cyclic variations in working fluid temperature are used in a refrigerator for cooling one end of the working fluid chamber in a manner well known in the art.
In either cae, the working fliuid is usually contained at a relatively high pressure and presents problems in terms of leakage both around the work piston through the dynamic seal between such piston and its cylinder wall, and also through various joints and connections in the machine housing to the outer atmosphere. The control of such leaks, particularly in cases where the working fluid is flammable, such as hydrogen, has been a source of continuing difficulty to which many solutions have been proposed.
The thermodyamic efficiency of such thermal machines is dependent in part on the mean pressure of the working gas and on the temperature differential between the hot and cold spaces of the displacer chamber. In an engine, greater efficiency can be obtained by supercharging the displacer chamber with working gas at very high pressures and then operating the engine at the greatest possible temperature differential between the hot and cold ends of the displacer chamber, thus heating the already highly pressurized working gas to very high temperatures. In practice however, the maximum safe operating pressures and temperatures of Stirling and similar engines are limited by the physical properties of the materials used in constructing the engine and on the construction and assembly techniques used.
Stirling cycle and similar machines are often designed along a main axis extending between a thermal end which receives the heat input and opposite work end at which work is either delivered in an engine or applied in the case of a refrigerator. Temperatures of engine components vary greatly at different points along this main axis, typically reaching extremes at the thermal end and graduating to near ambient temperature at the work end.
The bodies or housings of such machines are typically assembled in several axial body sections which are secured to one another by means of radial flanges on each body section. The flanges are bolted, screwed or clamped together to hold the various sections against the internal operating pressures of the machine and in combination with sealing rings or gaskets to make high pressure gas tight seals where needed to contain any pressurized fluids within the machine. Greater working fluid pressure require increasingly heavy cylinder walls at the same time that the strength of the engine body material is degraded as operating temperatures are increased. Metal alloys conventionally used in making machine body sections thus set upper limits to the working fluid pressures and temperatures which fall short of the operating parameters desirable for optimum machine efficiency.
Recent advances in material technology have produced new categories of materials, particularly ceramics, capable of withstanding substantially higher temperatures and pressures than metallic alloys normally used for engine components. While it would be advantageous to incorporate ceramic parts into thermal machines and particularly into external combustion engines in cases where operating temperatures exceed 1,100 degrees Centigrade (approximately 2,000 F.), difficulties exist in assembling a hybrid engine comprising both ceramic and metallic parts due to their dissimilar mechanical characteristics, particularly their varying coefficients of thermal expansion. In external combustion engines for example, it has been found advantageous to use ceramic material such as silicon carbide for the engine heater head while retaining metallic materials for the cooler sections of the engine. Ceramic materials while able to withstand substantial pressures, are more brittle than metallic components and thus do not flex readily under the compound stresses frequently imposed by fasteners, e.g. bolts, tie-rods, etc., normally used to assemble the axial sections comprising the machine body. The use of brittle materials for the heater head is complicated not only by the different mechanical properties at high temperatures, but also because the ceramic material is limited in the amount of working fluid pressure which it will safely tolerate due to its brittleness, particularly at high temperatures. Further, the physical properties of ceramic materials are less uniform and predictable than those of metallic alloys and the use of ceramics therefore calls for higher design safety margins. Even where the machine body sections are made of similar materials, e.g. all steel bodies, conventional assembly techniques call for relatively massive radial flanges on each body section which are bolted or clamped together. As operating pressures and temperatures are increased these flanges as well as the cylinder walls of the machine body must be made increasingly heavy, practical considerations ultimately limiting the maximum safe operating pressures and temperatures.
Another problem area in Stirling cycle machines has been adequate control over leakage of the pressurized working fluid. The working fluid is contained in a displacer chamber where it is subjected to a thermodynamic cycle with consequent expansion and contraction of the working fluid. The cyclical variations in working fluid pressure drive a work piston from which a work output is derived. In many previous engine designs, fluid leakage around the work piston has been a continuing source of difficulty, both in terms of contamination of the working fluid by extraneous fluids (e.g. hydraulic fluid pumped by the work piston) and also loss of pressurized working fluid through leakage around the work piston. One solution to this problem has been proposed by this applicant in U.S. Pat. No. 4,489,554, consisting of a compound work piston where two piston elements are connected by an axial, small diameter linkage which is easier to seal than the larger diameter piston and which may also form part of a pumping arrangement designed to recover leaking working fluid, either by returning the same to the displacer chamber or by feeding it to the engine burner for combustion.
Further improvement of Stirling cycle and similar machines is needed to overcome the aforementioned difficulties.